Rep. Steve King of Iowa, an outspoken conservative firebrand, will not seek the Republican nomination for his state's open Senate seat.

"I will not run for Senate in 2014," King said in a tweet Friday evening. "A Senate race takes me out of urgent battles in Congress that can't wait until 2015. Many thanks to all."

The prospect of King's candidacy to fill the seat of retiring Sen. Tom Harkin, D-Iowa, worried the Republican establishment in Washington. He was singled out by American Crossroads, the leading GOP outside advocacy group, as a candidate who would struggle to win a general election.

King pledged to ignore the criticism, but Republican aides said it became clear in recent weeks that fundraising would be a steep challenge, given his outspoken views on a variety of issues, particularly immigration.

Harkin is retiring after three decades in Washington.

The decision by King opened the door to other Republican candidates, but also suggested Rep. Bruce Braley, a Democrat, had the upper hand - for now, at least.